The rectangular shaped State flag of four components portions two parallelograms and two trapeziums is cut across by a parallelogram from a point 2.1/2" [6.35 cm] below the top left corner to a point of the same distance from the bottom corner on the right side. The standard measures 72" [182.88 cm] long by 36" [91.44 cm] wide. The parallelogram dividing the rectangle in this manner leaves two similar trapeziums at the top and bottom of the flag, with the lower trapezium assuming the inverted form of the upper trapezium. The parallelogram is again divided into two parallelograms of unequal depths, the upper being one inch [2.54 cm] wider that the lower which is 7.1/2" [19.05 cm] width.

The center of the flag is superimposed by the State crest [i.e. coat-of-arms] in red. The forefingers of the upright arms, which support the red crest, are equidistant at 61 cm from the left and right sides of the flag. (...)

Four colours are incorporated in the flag, red for the crest, yellow for the trapeziums, white for the upper parallelogram and black for the lower parallelogram.

Mark Sensen, 9 November 1997 and Dov Gutterman, 6 March 1999

Album des Pavillons 2000, page BR 2.1, says the usage is civil and state flag on land, civil ensign on vessels () and (I guess naval) jack. Smith 1980 designates it as but probably since then the use on land has changed and the blue state ensign has been introduced.Željko Heimer, 30 March 2001

The protocol manual for the London 2012 Olympics (Flags
and Anthems Manual London 2012) provides recommendations for national flag
designs. Each NOC was sent an image of the flag, including the PMS shades, for
their approval by LOCOG. Once this was obtained, LOCOG produced a 60 x 90 cm
version of the flag for further approval. So, while these specs may not be the
official, government, version of each flag, they are certainly what the NOC
believed the flag to be. For Brunei: PMS 102 yellow, 032 red and black. The
vertical version is simply the flag turned through 90 degrees clockwise.
Ian Sumner, 10 October 2012

[The image in Flaggenbuch 1939] is correct. I have a colour photocopy of the flag taken from U.K. Admiralty old book [Editor's note: probably Flags of All Nations 1955 or a previous edition].
Nozomi Kariyasu, 14 January 2000

There are a number of references to the white being wider than the black:

National Geographic 1934: no textual reference to the flag but in the illustration the black stripe is 2.5mm and the white stripe 4mm;

According to Grosvenor and Showalter in National Geographic 1934, "white is the colour of the Chief Minister of State, the Duli Pengiran Bendahara, and black represents the Second Minister, the Duli Pengiran
Pemancha". Perhaps the First Minister was entitled to a broader stripe than the Second Minister?
David Prothero, 24 January 2000

The current flag does not have 1:1 stripes. At least, according to the Brunei Government Official Website, the white stripe is 19.05 + 2.54 = 21.59 cm (i.e. 8.5") and the black one 19.05 cm (i.e. 7.5") which makes the ratio 17:15  the figure attributed by Nozomi Kariyasu to the 1906-1959 flag.
Santiago Dotor, 26 December 2000

The same explanation ["white is the colour of the Chief Minister of State, the Duli Pengiran Bendahara, and black represents the Second Minister, the Duli Pengiran
Pemancha"] is given by (for example) Crampton, Znamierowski and
Poels (and is used by myself on my own specification sheet), however, there
is also the addition that "(with the addition of yellow symbolizing the
ruler) these were the signatories of the Treaty with Britain in 1906". On
the other hand, an official publication entitled "The National Flag of
Brunei Darussalam" dated 3 August 1999 deletes any symbolism for the colours
and concentrates entirely on the arms added in 1959.

This leads me to suggest that the meaning of the colours given above is
accurate as to why the colours were chosen in 1906, and since this
significance has (apparently) not be replaced by newer version, that the
explanation remains valid.

The National Crest of Brunei Darussalam has developed from a Royal emblem and in its original form still maintains its status as one of the Royal emblems. The present National Crest was superimposed on the National flag after promulgation of the 1959 Brunei Constitution. The crest consists of:

Bendera: the flag

Payung Ubor-Ubor: the Royal Umbrella

The Bendera and Payung Ubor-Ubor have been Royal regalia's since the creation of the crest.

Sayap: the wing of four feathers

Symbolizes the protection of justice, tranquility, prosperity and peace.

Tangan or Kimhap: the hand

Signifies the Government's pledge to promote welfare, peace and prosperity.

Bulan: the crescent

The symbol of Islam, the national religion of Brunei Darussalam. The characters inscribed on the crescent (...) in yellow Arabic script is the state motto, which can be roughly translated: "Always Render Service with God's Guidance".(...)

The scroll beneath the crest reads "Brunei Darussalam" which means "Brunei, the abode of peace".